Pixel Coordinates of Image with Python
Eeeeeeh… that answer sounds horribly complicated to me!

Assuming you are getting mouse coordinates from event e (from invoke or modal callbacks of an operator), and assuming that operator runs into the main ('WINDOW' type) region of the UVEditor space, something like that should work (code not tested):

region = C.region
reg_x, reg_y = e.mouse_region_x, e.mouse_region_y
img_size = C.space.image.size

uv_x, uv_y = region.view2d.region_to_view(reg_x, reg_y)

img_x, img_y = uv_x * img_size[0], uvèy * img_size[1]